The present invention relates to an apparatus for measuring local cerebral blood flow.
Conventional apparatuses for measuring local cerebral blood flow include a single photon emission CT (SPECT) and a positron emission CT (PET). These apparatuses can measure r-CBF values. Among the conventional CT apparatuses, a CT apparatus for measuring local cerebral blood flow according to a tracer inhalation method has received a great deal of attention, since it has significant advantages as regards spatial resolution, quantum and cost performance when the local cerebral blood flow is reproduced as a functional image.
According to the tracer inhalation method, the concentration of a tracer in arterial blood and a time-concentration curve relative to the tracer concentration in the cerebral tissue are obtained by means of dynamic scanning using an X-ray CT apparatus while a nondiffusable gas such as Xe (Xenon) is inhaled as a tracer into the lungs. A distribution coefficient .lambda. between the blood and the cerebral tissue and the blood flow rate are calculated in each matrix of a slice, according to the Kety-Schmidt's formula, using the measured data. The calculated result is displayed as a functional image.
Tracer concentration Ca(t) in the arterial blood is calculated on the basis of a principle (Henley law) wherein a given concentration of tracer in the arterial blood has an equilibrium relationship with that in the alveolar air flowing from the alveoli of the lungs. Tracer concentration Ce(T) in the end tidal air, which corresponds to the alveolar air, is calculated to obtain the concentration Ca(t) present in the arterial blood.
U.S. patent application No. 746,523 now abandoned describes an apparatus using the above method. In this apparatus, exhaled air in cavities such as the windpipe, the nasal cavity, or the inner cavity of a breathing mask is exhausted into an expiratory tube. If the subject inhales air which includes a tracer, the rate of rise in the concentration of tracer in the expiratory tube increases. However, if the subject inhales only air or pure oxygen, the rate of fall in the concentration of tracer in the expiratory tube increases. An X-ray CT scan normally requires a period of 4 to 9 seconds. The tracer concentration is measured as an average of the number of breathing cycles which occur during the time of scanning a scan plane in the expiratory tube. Therefore, the rate of rise and fall in tracer concentration are further increased. For this reason, the changes with time (i.e., the increase add decrease) in the tracer concentration, measured by scanning the expiratory tube, are greater than those which occur in the actual end tidal air (i.e., the tail-end of the exhalation, from the end of the current exhalation cycle to the beginning of the next inhalation cycle. The end tidal air seems to correspond to the alveolar air). As a result, the cerebral blood flow cannot be accurately measured.